The present invention relates to a microwave directional coupler which is applicable to mixing, distribution and switching controls of microwave signals, and particularly to a distributed coupling type microwave directional coupler which has a pair of coupling line parts provided facing each other on both sides of a dielectric substance.
Circuit systems for processing microwave signals in most cases require directional couplers, for instance for mixing. As is well known, in these microwave regions, there are two methods for configuring directional couplers. Waveguides are used in one method, and strip lines or conductor lines are used in the other.
However, the latter type of directional couplers using conductor lines are believed to respond especially well to the latest needs for lightweight, compact electronic equipment, since they can be mounted on the same circuit board as the other circuit components.
Various types of such microwave directional couplers using conductor lines have been developed in the past, but those illustrated in FIGS. 1 through 7(B) may be regarded as typical. These couplers can be classified into two types: those of the continuous conductor pattern type, illustrated in FIGS. 1 and 2; and those of the quarter-wavelength distributed coupling type, illustrated in FIGS. 3 through 7(B).
The directional coupler 10 illustrated in FIG. 1 uses a plane type distribution line, in which all the conductor patterns are in ohmic contact at the D.C. level. It is generally called the "branch line" type. Described simply, the two pairs of terminals facing each other (pair 11 and 12 and pair 13 and 14) are each connected by single conductor lines 15 and 16, and the lines 15 and 16 are connected to each other at two places midway along their lengths, the interval separating them being equal to about a quarter wavelength. The connections are effected through two bridge lines (17 and 18) which face each other and which each have a length equal to about a quarter wavelength.
These conductor patterns are usually formed by patterning on a board (printed-wiring board) 19 made of a suitable dielectric material, and the desired circuitry is configured by mounting this directional coupler 10 on the same board as the other circuit systems, not shown in the figure.
In this directional coupler 10, signals inputted from the terminal 11 are outputted to the terminals 12 and 14, but not to the terminal 13. On the other hand, signals from the terminal 13 are outputted to the terminals 12 and 14, but not to the terminal 11.
The directional coupler 20 shown in FIG. 2 is called the "rat race" type. It is configured by patterning on a dielectric board 21 a circular conductor pattern 22 with a circumference equal to about 11/2 wavelengths, terminals 23 and 24 drawn out from two places facing each other in the radial direction, and two terminals 25 and 26 drawn out from points located about a quarter wavelength to the left or to the right from the terminals 23 and 24. In this directional coupler 20, microwave signals inputted from the terminal 23 are outputted to the terminals 24 and 25, but not to the terminal 26, and microwave signals from the terminal 24 are outputted to the terminals 23 and 26, but not to the terminal 25.
The directional couplers 10 and 20, shown in FIGS. 1 and 2, as mentioned above, both are formed so that their terminals are connected to each other perfectly by means of conductors. There are other types of directional couplers, called the quarter-wavelength distributed coupling type, in which the couplings are effected through distributed capacity.
FIG. 3 illustrates the basic configuration of a directional coupler 30 of the quarter-wavelength distributed coupling type. It has a pair of coupling lines 31 and 32, each having a length approximately equal to a quarter wavelength. They are located in parallel to each other as separated by a gap 33. Therefore, since the coupling lines 31 and 32 have their narrow sides facing each other, this coupler is sometimes called the "narrow side coupling" type. The terminal lines 34, 35 and 36, 37 are drawn out from both ends of each of the coupling lines. These conductor patterns are also formed by patterning on the surface of a suitable dielectric board 38.
In this directional coupler 30, microwave signals inputted from the terminal 34 are outputted to the terminals 35 and 36, but not to the terminal 37; and microwave signals inputted from the terminal 36 are outputted only to the terminals 34 and 37, but not to the terminal 35.
The directional coupler 40 shown in FIG. 4 is of one called the "overlay" type. The degree of coupling is improved by a short-circuit plate 44 which overlays the coupling lines 41 and 42 (corresponding to the lines 31 and 32 in FIG. 3) on the sectional area along line IV--IV, and the gap 43 between them.
The coupling lines 41 and 42 are formed by patterning on a dielectric board 45 and are embedded between the boards 45 and 46. 47 denotes the reference potential conductor surface (usually connected to a ground). This reference potential conductor surface 47 does not need to extend over the entire bottom surface of the board 45; it is sufficient if it covers only the part of the bottom surface corresponding to the coupling parts of the aforesaid coupling lines. This reference potential conductor surface is, of course, adopted in component structures which include the conventional examples given above.
The conventional example shown in FIG. 5 is a directional coupler 50 of the so-called tandem-connection type. The conventional example shown in FIG. 6 is a directional coupler 60 of the so-called interdigital type.
In each of these examples, coupling parts (55 or 6) having a tandem or interdigital shape for giving a predetermined orientation to the couplings between the four signal input/output terminals (51, 52 and 53, 54, or 61, 62 and 63, 64) are formed on the surface of a dielectric board (56 or 66), and these coupling parts are connected through bridge lines (57 and 58 or 67 and 68) formed with specific conductor patterns.
There are also conventional examples with a three-dimensional structure, in which horizontal layers are laminated in the vertical direction. The Triplate type directional coupler 70 shown in FIG. 7(A) and FIG. 7(B) is an example of such a coupler.
This directional coupler 70 is configured with a pair of dielectric boards 78 and 79 facing each other and separated from each other by a dielectric spacer board 80 which maintains them in this state. The dielectric boards 78 and 79 have on one surface pattern consisting of pairs of terminal lines (71a, 71b and 72a, 72b) and coupling lines 73 and 74; and on the other surface they have grounding conductor surface patterns 75 and 76.
Generally, the aforesaid three-layer structure is contained in a housing 81, the interior of which is sealed by clamping both halves 81a, 81b together with screws 82, as is shown in FIG. 7(B). If this housing 81 is made of metal, the aforesaid grounding conductor surface patterns 75 and 76 on the rear surfaces of the dielectric boards 78 and 79 are not needed.
As described above, there have been in the past various types of microwave directional couplers using conductor lines. Each type has its own advantages and disadvantages, and they all are laden with problems which need to be solved with respect to their performance as well as their physical construction.
First of all, let us mention a drawback which is believed to be common to all past configurations. That is, the material of the dielectric boards, which are necessary in directional couplers of this type, imposes severe restrictions on attaining the desired performance, not only because of the electrical characteristics which are required in these directional couplers, but also because of various other factors. For example, in the actual circuit designs in the past examples given above, a portion of the printed circuit board accommodating other peripheral circuits is commonly used for the dielectric board. That is, the conductor line patterns needed in a directional coupler of this type are formed integrally at the same time that the conductor patterns for the other circuit elements are formed on the printed-wiring board.
As a result, it is impossible to use expensive materials such as Teflon for the boards for directional couplers. Teflon, although expensive, has little dielectric loss and is suitable as the material for boards for directional couplers. However, such an expensive material cannot be used because the other circuit element parts calls for a large wasteful space. Generally speaking, there is a powerful demand for adopting cheaper elements. Consequently, a compromise solution was adopted in the past, using at the best materials such as glass epoxy resin or paper phenol which offered a tradeoff between the performance and costs of printed-wiring boards.
In addition to this common drawback, the directional couplers of the past still had problems unique to each type.
The directional couplers 10 and 20 of the "branch line" and "rat race" types illustrated in FIGS. 1 and 2 both have a drawback which cannot be ignored. That is, they both must occupy large two-dimensional spaces.
Concretely speaking, if the directional couplers 10 and 20 are to be used, for example, in microwave mixer stages of 1 GHz band, they will require, on their short sides alone, a rectangular plane space of at least 3 to 4 cm or more. This generally amounts to a more or less square space of 9 to 16 cm.sup.2 or even more.
This space is a quite large area. If we consider, for example, an application in which such microwave-using equipment is used in radar detectors or satellite communication receivers, we find that the circuit board area required by all the other circuits exclusive of these directional couplers will be on the order of 10 cm or less on their long sides. One can understand from this what a large area is occupied, comparatively, by the single circuit element of this mixer stage alone.
In this way, the size of the area occupied has recently become a bottleneck when electronic circuit systems using directional couplers of this type are put into practical application.
There still were drawbacks even when the conventional directional couplers of the types shown in FIGS. 1 and 2 were given -3 dB type designs. For example, the two outputs obtained by branching a single input tended to have differing intensities, and the bandwidths used were also narrow.
The bandwidths were improved in the quarter-wavelength type directional coupler 30 shown in FIG. 3, which had relatively broader bandwidths. However, there were other problems, such as a low degree of coupling and a limited degree of designing freedom.
In the "overlay" type directional coupler 40 shown in FIG. 4, the use of the short-circuit plate 44 not only broadened the bandwidths but also considerably improved the degree of coupling. Nevertheless, the embedded construction of the coupling lines 41 and 42 was impractical, being too difficult to fabricate.
With the tandem connection type directional coupler 50 shown in FIG. 5 and the interdigital type directional coupler 60 shown in FIG. 6, it was possible to improve the characteristics considerably. However, they also were impractical and difficult to fabricate because they required bridge lines (57 and 58, 67 and 68) spanning over the tops of the boards.
As for the Triplate type illustrated in FIG. 7(A) and FIG. 7(B), one would expect, in general principle, that extremely satisfactory results ought to be obtained. However, it is difficult to maintain the structure needed in order to obtain a good performance, and extremely advance technologies are required in order to position precisely all three layers. In addition, problems are presented by variations caused by the degree of tightening of the screws 82 when assembling the housing 81, and also by warping caused by the gaps between the boards 78, 79 at the parts where conductor patterns 73, 74 of the coupling lines are not present and the dielectric spacer board 80 between them.
Moreover, another drawback comes from the fact that the input/output terminals are located in the vacant spaces at the top, separated from the boards by intervals in the vertical direction. Therefore, there are difficulties in connecting them with the wiring parts of other peripheral circuit systems on the circuit board.